Air seeders are commonly towed by tractors to apply seed or fertilizer, or both simultaneously, to a field. An air seeder has as its central component a wheeled seed cart, which comprises one or more frame-mounted seed, tanks for holding product, generally seed or fertilizer or both. It is generally advantageous to tow a seed cart in combination with a tilling implement, one behind the other, to place the seed and fertilizer under the surface of the soil. Air seeders generally include a metering system for dispensing product from the tanks and a pneumatic distribution system for delivering the product from tank to soil.
The pneumatic distribution system of an air seeder generally utilizes a centrifugal fan to provide at least one airstream, which flows through the pneumatic distribution system to seed boots where product is deposited in the soil. Product is first introduced to the air stream by the metering system at a primary distribution manifold located below the metering system. Product is carried by the air stream through distribution lines to a series of secondary distribution manifolds, which in turn distribute product through distribution lines to seed boots mounted behind ground openers on the tilling implement so that the product may be evenly delivered to the span of ground (the "tillage") acted upon by the tilling implement.
In a double shoot configuration, equipment is provided for delivering two products separately through two different air streams (or sets of air streams) to the openers. Under many circumstances, it is desirable to have a mass flow rate and/or pressure in one set of air streams different from the rate of airflow in the other set of air streams. This is commonly achieved by inserting a device into the transition area between the blower and meter on the air cart which acts as a restricting element to divert a certain amount of airflow away from one set of air streams (and generally toward the other). Conventionally, the restricting element has taken one of a variety of forms, including, for example, a plate that is hinged at one end near the boundary between the two sets of air streams, a "guillotine" type door that can be lowered to restrict flow, a stationary plate inserted over the opening of the air stream, or a butterfly valve.
A problem common to the previously mentioned methods of restricting airflow is that the change in system response versus the change in position of the restricting element is decidedly non-linear throughout the range of movement of the restricting element. Therefore, an operator attempting to change mass flow rate or pressure in a set of primary tubes has difficulty determining the degree of movement of a restricting element required to effect the desired change.
It is therefore desirable to devise a pneumatic distribution system for an air seeder that enables an operator to correlate easily and reliably an adjustment made to the position of a restricting element to a resulting system response.